U.S. Pat. No. 6,183,199 B1 shows an example of a wall element for a hollow blade of a turbojet turbine, the element being pierced by a cooling channel of the above-specified type. In that example, the hole of the channel and its diffusion portion are made by electro-erosion in a single step, using a single electrode whose tip presents a front portion of a shape that corresponds to that of the hole and a rear portion of a shape corresponding to that of the diffusion portion of the channel.
An electrode of that type is described and shown in U.S. Pat. No. 4,197,443 to which reference is made in U.S. Pat. No. 6,183,199 B1. As can be seen, the shape of that electrode is particularly complex. In addition, in general manner, forming a channel by electroerosion using known methods remains an operation that is lengthy and expensive.